Specifically-trained physicians often implant a catheter or stent over a guidewire, proximate to, vascular and non-vascular occlusions; or to maintain patency of an anatomical lumen. To implant these devices, a physician must pass the device through vascular and non-vascular anatomical passages and cavities to reach the intended anatomical site which may exhibit an occlusion or an increased resistance when advancing the catheter or stent to the intended location.
One accepted method for passing these devices through anatomical passages includes placing the distal end (defined as the end that is farthest from the Physician) of a guidewire proximate to the anatomical site and advancing the catheter coaxially over the guidewire. In this method, the catheter is pushed from the proximal (defined as the end closest to the Physician) end of the guidewire to the distal end towards the anatomical site. The catheter is advanced either by hand (squeezing the catheter with an index finger and thumb and pushing along) or by using a relatively rigid plastic tube to push the catheter from behind.
In this method, the catheter must be pushed over the guidewire through long, sometimes tortuous, paths, reductions in lumen diameter and anatomical obstructions. Unfortunately, these obstacles require the physician to push the catheter or stent with increasingly excessive axial forces, and depending on the device design or the material from which the device is fabricated, buckling may occur.
Buckling of the device, due to excessive axial forces, is undesired as it may result in a damaged catheter, patient discomfort, accidental perforation of surrounding anatomy and prevention of catheter implantation to its intended location.
The axial load is the maximum force that may be applied to a catheter or stent when advancing the device into a patient before it begins to buckle. Therefore, the axial load is limited by the column strength of the device. This axial load is characterized by the Euler Equation for a simply supported device column under an external axial load (F); wherein (E=modulus of elasticity of the device; I=is the moment of inertia of the cross section of the device; and L=column length of the device). Accordingly, the buckling load of a catheter or stent can be determined by:F=((E)*(I)(3.14)2)/(L2)
With respect to the Euler Equation, if Length (L), and Moment of Inertia (I) are constant, only a change in materials with an increase in modulus of Elasticity (E) will increase the axial load capacity of a catheter or stent. For example, using the above Euler Equation, pushing a catheter with a length of (0.5 inches (L=0.50 inch)); compared to a catheter with a length of (12 inches (L=12.0 inch)) exhibits a profound effect on the axial load capacity (F). Considering that (L) is raised to the power of (2); it can be seen that (0.50 inches)2 versus (12 inches)2=0.25 versus 144; or a factor of over 500:1. Thus, devices with shorter columns will have a much greater axial load capacity resisting bucking. Unfortunately, shorter catheters are not practical; nor are they typically used in conventional applications where pushing a catheter is the method of delivery.
Multiple techniques have been developed by physicians to prevent buckling of the catheter during implantation. Many of the techniques mentioned below are often combined to reduce catheter buckling.
One technique is for physicians to apply only small amounts of axial force to the catheter either manually (by hand) or with a plastic tube (pusher catheter). This small axial force results in advancing the catheter several millimeters at a time. While this slow advance of the catheter does provide immediate tactile feedback to the physician should the catheter confront an obstruction during advancement, the process is long and tedious. In addition, once the catheter reaches the vascular occlusion at the implantation site, additional axial forces still must be applied in order to push the catheter across the occlusion. This increased axial force often causes the catheter, especially smaller diameter devices, to buckle.
An additional technique to prevent buckling of a catheter during implantation is to use a catheter or stent that is coated with layers of a lubricous polymer. These lubricous polymer coatings, which are typically are very thin, and relatively fragile, reduce the coefficient of friction of the device, which results in a reduced axial load. When these coated devices are inserted and advanced through anatomical paths, they encounter boundaries, such as vessel walls. These coated devices, which are flexible enough to bend tangentially at these boundaries, continue advancing along the anatomical path; however, these tangential bends may add external pressure to the coating at the points of contact. These external pressure points on the device contribute to the aqueous media being squeezed from the coating, much like squeezing water from a sponge. Ultimately, once the coating is compromised, any further contact between the device and anatomical boundaries at these tangential bends will exhibit an increased level of relative friction, such as dry on dry surfaces that results in increased axial loads required to push or advance the catheter along. In addition, this high friction may result in patient discomfort.
A further technique to prevent buckling of the catheter during implantation is to fabricate whole or portions of catheters and stents from moderate to rigid polymers. Similarly, a given thickness of rigid polymer can be extruded while a softer layer is coaxially extruded over or under the more rigid layer. Additionally, a layer of braiding material, either polymeric and or metallic, may be incorporated into the design of a catheter or stent whereby the braid is deposited or sandwiched between layers of catheter material. While these designs are more effective they result in undesirable and uncomfortable products as well as increased manufacturing costs.
Catheters or stents may also be manufactured with rigid portions, such as rigid proximal ends. These more rigid materials often cause patient discomfort and in many cases result in complications due to their increased ability to perforate surrounding anatomy during implantation.
Conventional delivery methods may also include coaxially placing and advancing soft catheters or stents within hollow rigid sheaths, avoiding the need to develop or design catheters that by themselves exhibit superior handling.
In any case, using a sheath or manufacturing a catheter out of more rigid material or incorporating a layer of braiding material are only attempts to increase the column strength of a catheter or stent whereby axial forces and corresponding push-ability can be maximized.
However, using a more rigid material either as part of the catheter, stent or sheath still results in patent pain and or discomfort.
As mentioned previously, many of the techniques mentioned above are often combined to reduce catheter buckling. For instance, in an attempt to augment conventional sheath-over-catheter delivery methods, catheters may be coaxially braided, a more rigid layer maybe integrated, and/or the device may be coated with layers of lubricous polymer. However, even a combination of these techniques results in patient discomfort, slow advancement and eventual buckling of the catheter or stent when increased axial loads are required to advance into or through anatomical passages.
Therefore, embodiments of the invention establish a novel method for delivery of catheters and stents that overcomes the buckling phenomena.